Heat-sensitive recording materials that utilize a thermal color-forming reaction between a leuco dye and a developer are well known. Such heat-sensitive recording materials are relatively inexpensive, and recording devices are relatively compact and easily maintained. Therefore, such heat-sensitive recording materials have been widely used as recording media for facsimile machines, various labels, and other types of output, and also as recording media for the printers of medical diagnostic devices for ultrasound imaging, X-ray imaging, etc.
Heat-sensitive recording materials containing synthetic paper having a multiple-layer structure or a biaxially oriented thermoplastic resin film optionally containing an inorganic pigment as a support are currently used for the printers of medical diagnostic devices, in which uniform recorded images and high resolution are required. In recent years, there has been increasing demand for a heat-sensitive recording material that has excellent gradation reproducibility from low to high density and that can provide a high-quality recorded image with a high gloss that is equivalent to a silver halide photograph.
To enhance printing preservability and printing suitability, Patent Literature (PTL) 1 proposes a heat-sensitive recording material comprising a heat-sensitive recording layer; an intermediate layer containing an aqueous resin that is formed on the heat-sensitive recording layer; and a protective layer containing a resin cured by electron beam irradiation that is formed on the intermediate layer; and further comprising an overcoat layer containing a pigment that is formed on the protective layer. However, a protective layer containing a resin cured by electron beam irradiation is generally highly hydrophobic and has a low wetting tension. Therefore, it is extremely difficult to uniformly apply a pigment dispersion containing water as a medium to the protective layer to form a coating film thereon. If the thickness of the overcoat layer is increased to provide a uniform coating film, gloss, recording density, coating adhesion, etc., are sharply reduced.
To form a uniform coating film with enhanced gloss, Patent Literature (PTL) 2 proposes a heat-sensitive recording material comprising a heat-sensitive recording layer, a first intermediate layer, a second intermediate layer containing a resin cured by the irradiation of an electron beam, and an overcoat layer mainly consisting of a pigment and an aqueous resin, which are successively layered in this order; and Patent Literature (PTL) 3 proposes a heat-sensitive recording material comprising a heat-sensitive recording layer, a first intermediate layer containing an aqueous resin, a second intermediate layer cured by ionizing radiation, and an uppermost layer containing an aqueous resin, which are sequentially formed in this order. However, in both of the above recording materials, the outermost layer is first formed on a highly smooth base material that is different from the support used in the heat-sensitive recording material, and then the outermost layer is transferred to the heat-sensitive recording layer side. Accordingly, partially uneven transfer results in poor recording surface quality, recorded image quality, gloss, etc. Moreover, the complicated steps of such a method lead to poor productivity, and entail a disposal problem for the highly smooth base material, etc. Furthermore, due to high smoothness of the outermost layer, soiling, such as that by sebum and perspiration, which adheres to the surface of the heat-sensitive recording material when the surface is touched with a bare hand, is prone to be transferred to the thermal head surface of a printer, thus causing printing failures, such as printing scratches and white streaks.
To prevent components of the heat-sensitive recording material from adhering as residues to the thermal head surface, Patent Literature (PTL) 4 proposes a heat-sensitive recording material comprising a protective layer containing a silicone modified polymer and inorganic ultrafine particles having a particle diameter of 0.1 μm or less. However, because the use of a silicone modified polymer lowers the wetting tension of the protective layer surface, dust and other foreign matter that enter and adhere to the protective layer of the heat-sensitive recording material, particularly soiling substances such as sebum and perspiration that adhere when the surface is touched with a bare hand, are prone to be transferred to the thermal head during recording. Furthermore, due to increased lubricity between the protective layer surface and the thermal head during printing, soiling, etc., once transferred cannot be easily scratched off. As a result, soiling such as sebum and perspiration tends to accumulate as a residue on the thermal head surface, and most likely causes printing failures, such as scratches and white streaks. In addition, once such a problem occurs, it is difficult to resolve. Furthermore, the application of a coating composition containing inorganic ultrafine particles with a particle diameter of 0.1 μm or less tends to result in uneven coating, etc.; therefore, satisfactory results are not always obtained.
To obtain a high-quality recorded image with a high gloss, Patent Literature (PTL) 5 proposes forming, on an intermediate layer containing an aqueous resin, a protective layer comprising an electron-beam-curable resin composition cured by electron beam irradiation. However, when the protective coating layer surface has a gloss at 75° of not less than 90% and a gloss at 20° of not less than 35% as measured in accordance with JIS P 8142-1993, soiling such as that by sebum and perspiration that adheres to the heat-sensitive recording material when it is touched with a bare hand sticks to the thermal head during recording, thus causing recording failures such as streaks. Therefore, no heat-sensitive recording material that is excellent in terms of both running performance in the recording and gloss has yet been obtained.
Patent Literature (PTL) 6 proposes a method for manufacturing a heat-sensitive recording material, comprising forming on a support a heat-sensitive recording layer and a first intermediate layer; applying to the first intermediate layer a second intermediate coating composition containing an ionizing-radiation-curable compound to form a second intermediate coating layer surface; forming an aqueous resin-containing uppermost layer on a film that is different from the support; adhering the second intermediate coating layer surface to the uppermost layer and curing the second intermediate coating layer by irradiation with ionizing radiation; and then peeling off the film, thereby providing a heat-sensitive recording material that is excellent in terms of running performance in the recording, recorded image quality, and gloss. However, considering the quality deterioration caused by uneven peeling or the necessity of using a film in a step of the manufacturing method, there is room for further improvement in productivity.